Why would AWS provide a feature that makes additional transaction and storage charges (as well as subsequent reads to see which is the correct version) irrelevant?
S3 (and most AWS services) are extremely price elastic; i.e., the lower you make them cost, the more people use it (a la electricity.) That's why they've done stuff like drop from 50ms billing to 1ms billing, etc.
They could still offer it as a client library feature, just tell the users what kind of r/w amplification and guarantees they can expect, and it's something they could optimize later or not.
Conditional PUTs aren't possible with versioned objects, but if you desire immutability, then they do help.
To implement serialisation, (as opposed to using Conditional PUTs) one could implement a ledger on top of versioned buckets with LegalHolds: Basically, the object versions part of the main chain are LegalHolded whilst other versions are reconcilled (rebased) onto main and later deleted. Tricky to implement, for sure, compared to say, maintaining a journal in DynamoDB to track PUTs to maintain serial integrity.